National pipe thread
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National Pipe Thread Taper (NPT) is a U.S. standard for tapered threads used on threaded pipes and fittings. In contrast to straight threads that are found on a bolt, a taper thread will pull tight and therefore make a fluid-tight seal.
Types and terminology 
Sometimes NPT threads are referred to as MPT ('Male Pipe Thread'), MNPT, or NPT(M) for male (external) threads; and FPT ('Female Pipe Thread'), FNPT. An equivalent designation is MIP (Male iron pipe) and FIP (Female iron pipe). Also the terms NPS and NPSM are sometimes used to designate a straight, not tapered, thread. This should not be confused with NPS, meaning Nominal Pipe Size.
NPTF is pipe thread "fuel" often used in hydraulics. A taper is cut on the id of male pipe to accommodate metal to metal seals using swivels. NPSH is straight thread using the same pitch, thus NPSH female fittings are often used on NPT male fittings to have hand tight gasket sealed fittings to adapt to other thread types. A good example is adapting to NST for fire service.
Sometimes these terms are used:
- MIP: stands for Male Iron Pipe, or Male International Pipe, or MPT Male Pipe Thread. It is a term for pipe fittings.
- FIP: stands for Female Iron Pipe, or Female International Pipe, or FPT. It is a term for pipe fittings that MIP fittings fit into.
A female iron connection has a tapered thread, which thins out to the end of the pipe. As the fitting is tightened, the ever-decreasing thread depth means that the connection becomes watertight. To properly seal the fitting, paste thread sealant or PTFE (polytetrafluoroethylene) tape more commonly called Teflon tape, wrapped around the thread is required.
MIP fittings are usually found in thicker copper pipe, iron pipe, steel pipe, gas pipe, gas stove, gas oven, gas heater, and gas water heater connections.
The taper rate for all NPT threads is 1⁄16 per inch (3⁄4 inch per foot) measured by the change of diameter (of the pipe thread) over distance. The angle between the taper and the center axis of the pipe is 1° 47′ 24″ (1.7899°). Commonly-used sizes are 1⁄8, 1⁄4, 3⁄8, 1⁄2, 3⁄4, 1, 1 1⁄4, 1 1⁄2, and 2 inch, appearing on pipes and fittings by most U.S. suppliers. Sizes smaller than 1⁄8 inch are occasionally used for compressed air, while sizes larger than 2 inches are uncommon, due to the use of alternative methods of joining that are used with these larger sizes. NPT is defined by ANSI/ASME standard B1.20.1.
Nominal Pipe Size is loosely related to the inside diameter of Schedule 40 pipe. Because of the pipe wall thickness, the actual diameter of the threads is larger than the NPS, considerably so for small NPS. Other schedules of pipe have different wall thickness but the OD (outer diameter) and thread profile remain the same, so the inside diameter of the pipe is therefore different from the nominal diameter.
Threaded pipes can provide an effective seal for pipes transporting liquids, gases, steam, and hydraulic fluid. These threads are now used in materials other than steel and brass, including PTFE, PVC, nylon, bronze, and cast iron.
The taper on NPT threads allows them to form a seal when torqued as the flanks of the threads compress against each other, as opposed to parallel/straight thread fittings or compression fittings in which the threads merely hold the pieces together and do not provide the seal. As the thread body is tapered (0.75 in/ft) a larger diameter keeps compressing into a smaller diameter and finally forms a seal (no clearance remains between the crests and roots of the threads because of the taper). This means that NPT fittings should be burr free and lubricated using a lubricating material like lubricating paste or tape. The use of tape also helps to limit corrosion on the threads, which otherwise can make future disassembly nearly impossible.
A semi-compatible variant called National Pipe Taper Fuel (NPTF), also called Dryseal American National Standard Taper Pipe Thread, defined by ANSI B1.20.3, is designed to provide a more leak-free seal without the use of teflon tape or other sealant compound. NPTF threads are the same basic shape but with crest and root heights adjusted for an interference fit, eliminating the spiral leakage path.
|Nominal pipe size (in)||Pipe outer diameter||Threads per inch||Thread pitch|
|1⁄16||0.3125 in (7.94 mm)||27||0.03704 in (0.94082 mm)|
|1⁄8||0.405 in (10.29 mm)||27||0.03704 in (0.94082 mm)|
|1⁄4||0.540 in (13.72 mm)||18||0.05556 in (1.41122 mm)|
|3⁄8||0.675 in (17.15 mm)||18||0.05556 in (1.41122 mm)|
|1⁄2||0.840 in (21.34 mm)||14||0.07143 in (1.81432 mm)|
|3⁄4||1.050 in (26.67 mm)||14||0.07143 in (1.81432 mm)|
|1||1.315 in (33.40 mm)||11 1⁄2||0.08696 in (2.20878 mm)|
|1 1⁄4||1.660 in (42.16 mm)||11 1⁄2||0.08696 in (2.20878 mm)|
|1 1⁄2||1.900 in (48.26 mm)||11 1⁄2||0.08696 in (2.20878 mm)|
|2||2.375 in (60.33 mm)||11 1⁄2||0.08696 in (2.20878 mm)|
|2 1⁄2||2.875 in (73.03 mm)||8||0.12500 in (3.17500 mm)|
|3||3.500 in (88.90 mm)||8||0.12500 in (3.17500 mm)|
|3 1⁄2||4.000 in (101.60 mm)||8||0.12500 in (3.17500 mm)|
|4||4.500 in (114.30 mm)||8||0.12500 in (3.17500 mm)|
|5||5.563 in (141.30 mm)||8||0.12500 in (3.17500 mm)|
|6||6.625 in (168.28 mm)||8||0.12500 in (3.17500 mm)|
|8||8.625 in (219.08 mm)||8||0.12500 in (3.17500 mm)|
|10||10.750 in (273.05 mm)||8||0.12500 in (3.17500 mm)|
|12||12.750 in (323.85 mm)||8||0.12500 in (3.17500 mm)|
|14||14.000 in (355.60 mm)||8||0.12500 in (3.17500 mm)|
|16||16.000 in (406.40 mm)||8||0.12500 in (3.17500 mm)|
|18||18.000 in (457.20 mm)||8||0.12500 in (3.17500 mm)|
|20||20.000 in (508.00 mm)||8||0.12500 in (3.17500 mm)|
|24||24.000 in (609.60 mm)||8||0.12500 in (3.17500 mm)|
Thread form 
Nominal pipe sizes 
||This section may require cleanup to meet Wikipedia's quality standards. (September 2010)|
See also 
Further reading 
Oberg, Erik; Franklin D. Jones, Holbrook L. Horton, and Henry H. Ryffel (2000). In ed. Christopher J. McCauley, Riccardo Heald, and Muhammed Iqbal Hussain. Machinery's Handbook (26th edition ed.). New York: Industrial Press Inc. ISBN 0-8311-2635-3.